Ore-forming Processes Research Articles

Formation mechanism of boehmite and diaspore in karstic bauxites: trace element geochemistry in source materials using large sample geochemical dataset and random forest model

Abstract Boehmite and diaspore are the two economic ore minerals of karstic bauxites. Although their genesis has been studied from different perspectives, their formation mechanism is controversial. Random forest (RF) model of machine learning was employed to extract the combining characteristics of trace elements in boehmite-type bauxite (BTB) and diaspore-type bauxite (DTB). The BTB predominantly exhibits higher median concentrations of Co, Ni, V, and Cr, while the DTB exhibits a more significant enrichment in elements of U, Hf, Th, and Zr. Combining the characteristics of La/Yb and Ga/Al ratios, it is found that disparities between BTB and DTB are mirroring to those between basic rocks and intermediate-felsic rocks. Furthermore, the Zr-Cr-Ga diagram and Ni/Zr versus Cr/Zr plot were utilized to examine the correlation between karstic bauxite (BTB or DTB), lateritic bauxite, and their respective weathered parent rocks. It is found that BTB exhibits consistent characteristics with lateritic bauxite weathered from basic rock and its parent rocks. Similarly, DTB displays consistent characteristics with lateritic bauxite weathered from intermediate-felsic rock and its parent rocks. Through studying the relationship between Ni content and Fe3+/Fe2+ ratios, it has been discovered that the presence of trace elements like Ni in source materials can affect or regulate ore-forming process, ultimately driving the transformation of gibbsite into either boehmite or diaspore. Consequently, the formation of BTB and DTB is significantly influenced, or even governed, by the composition of their source materials. Our study initially highlights a novel insight into the significant impact of source material’s geochemical composition on the formation of boehmite and diaspore, which is associated with weathered igneous rocks categories.

Geological Characteristics, Ore-forming Processes, and Mineral Resources of Rare Earth Element Deposits in Mongolia

Geological Characteristics, Ore-forming Processes, and Mineral Resources of Rare Earth Element Deposits in Mongolia

LA-ICP-MS Trace Element Characteristics and Geological Significance of Stibnite in the Zhaxikang Pb–Zn–Ag–Sb Deposit, Southern Tibet, SW China

Discovered within the North Himalayan Metallogenic Belt (NHMB), the Zhaxikang Pb–Zn–Ag–Sb deposit stands as the sole super-large scale ore deposit in the region. This deposit holds significant quantities of Pb and Zn (2.066 million tons at 6.38% average grade), Ag (2661 tons at an average of 101.64 g/t), and Sb (0.235 million tons at 1.14% average grade), making it one of China’s foremost Sb–polymetallic deposits. Stibnite represents the main carrier of Sb in this deposit and has been of great attention since its initial discovery. However, the trace element composition of stibnite in the Zhaxikang deposit has not yet been determined. This study carried out an analysis of the distribution patterns and substitution processes of trace elements within stibnite gathered from the Zhaxikang deposit, aiming to provide crucial information on ore-forming processes. Utilizing high-precision laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we discovered that the studied stibnite is notably enriched in arsenic (~100 ppm) and lead (~10 ppm). Furthermore, the notably consistent time-resolved profiles suggest that elements such as Fe, Cu, As, In, Sn, Hg, and Pb predominantly exist as solid solutions within stibnite. Consequently, it is probable that the enrichment of Cu, Pb, and Sn in stibnite is due to isomorphic substitution reactions, including 3Pb2+↔2Sb3+, Cu+ + Pb2+↔Sb3+, and In3+ + Sn3+↔2Sb3+. Apart from that, Mn, Pb, and Hg with the spiky signals indicate their existence within stibnite as micro-inclusions. Overall, we found that the trace element substitutions in stibnite from the Zhaxikang Pb–Zn–Ag–Sb deposit are complicated. Incorporations of trace elements such as Pb, Cu, and In into stibnite are largely influenced by a variety of factors. The simple lattice structure and constant trace elements in studied stibnite indicate a low-temperature hydrothermal system and a relatively stable process for stibnite formation.

Spatial and temporal reconstruction of postcollisional potassic rocks: Implications for mantle flow beneath the SE Tibetan Plateau

Postcollisional potassic magmatism occurs throughout the Alpine-Himalayan belt, providing a crucial record of crust-mantle interaction and associated ore-forming processes in a collisional orogen. Unravelling its spatial and temporal record is often complicated by intense tectonic deformation, including strike-slip faulting. New Hf-Nd isotope mapping of two Cenozoic potassic rock assemblages along the Ailaoshan−Red River shear zone from the SE Tibetan Plateau, together with Sr-Nd-Hf isotope evidence, establishes that these two segments of potassic magmatism were originally juxtaposed prior to strike-slip faulting. By reconstructing the spatiotemporal distribution of the potassic rocks, we identified a zone of mantle upwelling and lateral flow in response to India-Asia collision. This activity matches the distribution of porphyry Cu-Au and carbonatite-associated rare earth element deposits in the area, demonstrating the significant contribution of mantle materials to the metallogenic processes in collisional settings.

Ore-forming processes of giant carbonate-hosted Zn Pb deposit and Ge enrichment mechanism in Zhugongtang, Guizhou Province, China: Constraints from trace element and isotopic compositions of sulfides

Ore-forming processes of giant carbonate-hosted Zn Pb deposit and Ge enrichment mechanism in Zhugongtang, Guizhou Province, China: Constraints from trace element and isotopic compositions of sulfides

Ore-forming process of the Saibagou gold deposit in the Northern Qaidam Orogen: Evidence from fluid inclusions, D-O isotopes and pyrite geochemistry

The northern Qaidam Orogen that hosts over 100 gold deposits is an important gold mineralization belt in Qinghai Province (western China). However, ore-forming processes and genesis of these gold deposits remain controversial. We analyzed the fluid inclusions (FIs), D-O isotopes and pyrite trace element compositions of the Saibagou gold deposit in the Tommorrit-Saibagou area and discussed the genesis of this deposit. These results show that the Saibagou gold ores contain three types of FIs: aqueous, aqueous-CO2, and pure CO2 type. The ore-forming fluids in each stage changed from an early medium temperature and low-salinity H2O-NaCl ± CO2 system to a late low-temperature and low-salinity H2O-NaCl system. The D-O isotopes show that the ore-forming fluids are the mixed fluids of metamorphic water and meteoric water. In the main-ore stage, the ore fluids are mainly metamorphic and minor magmatic water sourced, with increasing meteoric water incursion toward the late-ore stage. The Au and other ore-forming elements (such as Ag, Cu, Pb, Zn, Te) are positively correlated and are mainly enriched in pyrite in the stage II. We suggest that the Saibagou is an orogenic gold deposit and is closely related to the Late Silurian northward subduction and hyperplastic orogenesis in Qaidam Block.

Role of As in the formation of giant Au deposits: Insights from sulfur isotope and geochemistry of pyrite from the Shuangwang Au deposit, West Qinling, central China

The Shuangwang Au deposit (SWGD), located in the Feng-Tai ore district, is one of the largest Au deposits in the West Qinling orogen, central China. Au orebodies in this deposit are mainly hosted in the Upper Devonian Xinghongpu Formation. The source of the fluids and metals, ore-forming processes and the enrichment mechanism of Au in this ore deposit remain equivocal. Here we present results from geochemical studies of pyrite from the SWGD to address these issues through the application of Scanning electron microscopy (SEM), electron probe microanalysis (EPMA), in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and LA-MC-ICP-MS analyses. Based on detailed petrographic studies of the mineralization and host rocks, we identify seven generations of pyrite. Among these, the pre-ore syn-sedimentary pyrite (Py0) is characterized by relatively elevated concentrations of As, Co, Ni and Au compared to the Py1 in the albite-ankerite veins and Py2 in the quartz-ankerite-pyrite veins, and has positive δ34S values of + 6.00 to + 10.48 ‰, whereas Py3 forms as veinlets or aggregates in the pyrite-tourmaline veins and is enriched in As, Co, Cu, Se and Au. Py4 occur as coarse-grained subhedral to euhedral crystals or aggregates in the ankerite-pyrite veins and exhibit oscillatory zones. Py5 comprises fined-grained subhedral to anhedral grains in the fluorite-ankerite-pyrite cements, and is relatively depleted in trace elements when compared to the other pyrite types. The δ34S values of the different types of pyrite from the SWGD have significant differences, indicating a multiple and mixed source for sulfur and, by inference, for the ore-forming fluid(s). Two periods of Au mineralization are identified in the SWGD: the early NW-trending mineralization sourced from the metamorphism of the metasedimentary sequences in a compressional environment and the late NE-trending mineralization probably exsolved from an unexposed and coeval granitoid pluton at depths in a post-collisional extensional setting. The coupled geochemical behavior of Au and As can be explained by mixing-induced cooling. Our results also indicate that arsenic might have played an important role in the enrichment of Au and the formation of this hydrothermal Au deposit.

Differences in Sm/Nd ratios between early magmatic and late sulfides: The role of fluids and Nd mobility

This paper presents the results of a SmNd study of sulfide minerals and whole-rock samples from Cu-Ni-PGE layered complexes of the Fennoscandian Shield. Syngenetic (early) and epigenetic (late) sulfides were analyzed in each complex using the SmNd method. Late sulfide minerals with low Sm/Nd values (the 147Sm/144Nd ratio is often in the range of 0.02–0.07) are associated with an increased mobility of Nd relative to Sm, resulting in a relative excess of Nd compared to Sm in these sulfides. Simultaneously, early sulfides, which are deposited during the magmatic stage of ore formation, typically exhibit higher Sm/Nd values (the 147Sm/144Nd ratio is frequently above 0.07). Additionally, SmNd isotope data for sulfide minerals were used to date ore-forming processes in two Cu-Ni-PGE complexes—Nyud-II (Monchegorsk area, Russia) and Ahmavaara (Finland). The SmNd ages of syngenetic and metamorphic ore from these complexes were determined. Syngenetic ores formed at 2496 ± 36 Ma (Nyud-II) and 2441 ± 93 Ma (Ahmavaara), while metamorphic ores formed at 1940 ± 32 Ma (Nyud-II) and 1904 ± 24 Ma (Ahmavaara). Thus, SmNd isochrons yield the timing of sulfide mineralization and its relationship with the ages of the rocks containing it, while Sm/Nd ratios in sulfides help understand the processes of ore formation. A comprehensive analysis of the full isotopic dataset (this study and other published data) showed the potential of using SmNd isotope data to trace the sequence of sulfide mineralization, which has been confirmed for some hydrothermal deposits. However, this sequence has not been confirmed for magmatic sulfides; this opens up the possibility for further research.

Mineral composition of xenoliths of transformed tholeiitic basalts in ore-hosting rocks of the Rudnogorskoe iron ore deposit, Eastern Siberia

Research subject. Variously-altered tholeiitic basalt xenoliths in ore-hosting rocks from the Rudnogorskoe iron ore deposit, the Angaro-Ilim region of Eastern Siberia. Aim. To identify the sequence of mineral transformations during the formation of magnetite ores. Materials and methods. The mineral composition of weakly-altered and hematitized xenoliths of tholeiitic basalts in skarned rocks and relics of basaltic hyaloand lithoclasts in varying degrees magnetitized volcaniclastites were studied. Minerals were identified using a powder X-ray diffractometer with the determination of the quantitative ratios of mineral phases by SHIMADZU XRD-6000 and DRON-2.0 diffractometers and using the microscopic (Olympus BX51) and electron microscopic (Tescan Vega 3 sbu with an Oxford Instruments Xact energy-dispersive analyzer) and IR spectroscopic (Spectrum One IR Fourier-spectrometer and a Multiscope microscope, PerkinElmer) research methods. Results. In xenoliths of weakly-altered tholeiitic basalts, volcanic glass is smectitized and partially replaced by secondary aggregates of chlorite and carbonate. In hematitized xenoliths, the smectite-hematite mineral association contains skarn epidote and garnet. Smectite aggregates, partially transformed into magnetite mass, are present in the magnetitized volcaniclastites. The studied smectites are classified as saponite according to the obtained values of basal reflections d001 in the range of 14.76–15.23 Å and calculated crystal chemical formulas. The differences in the morphology, chemical composition, and IR spectrometric characteristics of smectites reflect the varying degrees of transformation of the tholeiitic basalts in multi-stage ore-forming processes.

Translating mineral systems criteria into a prospectivity model for IOCG deposits in the Kolari region, Finland

The mineral system approach facilitates mapping the fingerprints of geological processes in ore formation using mineral prospectivity modeling (MPM). It aims to reduce the time and cost of exploration as a priority in the mineral exploration industry. Finland, which is covered with thick soil, dense vegetation, and snow in winter and thus has limited outcrops, serves as a suitable testing ground for this method. In this contribution, MPM was employed to predict favorable targets for Fe-Cu-Au (IOCG) deposits in the Kolari region, northwestern Finland. The mineral system components ore metal and sulfur sources, pathways, energy sources/drivers, traps, and the geological factors influencing the ore-forming processes were translated into mappable criteria and were considered for a regional-scale analysis. Knowledge-driven, fuzzy logic overlay, fuzzy inference system, and geometric average methods integrate evidential layers derived from geological (scale-free geology map), geochemical (till and bedrock sample geochemistry), and geophysical data (magnetic, radiometric, electromagnetic measurements, and gravity worms). Subsequently, the data were modeled using a combined conceptual/empirical approach, i.e. fuzzified evidential layers and logistic regression. In the prospectivity models, existing IOCG deposits are consistently placed within high-favorability areas. In addition, new exploration targets were identified. The models were validated against known IOCG deposits using Receiver Operating Characteristic (ROC) analysis and average Area Under the Curve (AUC) with values of final prospectivity models consistently reaching scores > 0.8. This indicates a favorable outcome of the application of MPM, making the approach applicable to other regions and ore deposit types. A majority voting ensemble technique was employed to combine the favorable areas of each prospectivity model. Then a confidence index was adopted reducing uncertainty linked to the models generated. These outputs consistently facilitated the identification of exploration targets more reliably.

In-situ trace element and sulfur isotope compositions of Multi-Stage sulfides in the Buzhu orogenic gold (Antimony) Deposit, southern Tibet: Implications for the metallogenic process

The Buzhu gold (antimony) deposit, located within the gold-antimony polymetallic belt of southern Tibet in the Tethys Himalayan belt, is a recently discovered medium-sized gold polymetallic deposit. The orebody consists primarily of gold-bearing quartz sulfide veins. Currently, the metallogenic mechanism of the deposit remains largely unexplored. Utilizing results from the mineralogical observation of principal ore minerals, LA-ICP-MS trace element analysis, and in-situ sulfur isotope analysis of pyrite, investigations reconstruct the evolution process of pyrite-arsenopyrite and constrain the metallogenic process of the Buzhu gold (antimony) deposit. It has been discovered that the gold-bearing sulfides in the Buzhu gold (antimony) deposit are primarily pyrite and arsenopyrite. Pyrite can be categorized into six generations: framboidal pyrite Py0, dissolution cavity pyrite Py1a (some exhibiting oscillatory zoning), disseminated pyrite Py1b, euhedral pyrite Py2a, irregular pyrite Py2b, and Py3. Arsenopyrite, on the other hand, can be classified into three generations: early arsenopyrite Apy0, disseminated arsenopyrite Apy1, and euhedral Apy2. The analysis of δ34S in pyrite throughout the ore-forming process indicates sulfur derives from multiple sources, with Py0 indicative of a biological sulfur source. The evolutionary process of pyrite, supported by trace element analysis, suggests that the remaining pyrite derives from sedimentary fluid transformation, with sulfur primarily sourced from metamorphic sediments. Elemental spectrum analysis and BSE observations indicate that the Buzhu gold (antimony) deposit primarily undergoes two distinct stages of mineralization: the initial stage is the gold-antimony mineralization phase, during which antimony precipitates in the fissures of Py1a as stibnite, while gold is primarily found in Py1-Apy1 as invisible gold, with the highest gold content in Apy1 (28.20–60.13 ppm, average 40.51 ppm). The subsequent stage of mineralization is characterized by gold mineralization, with gold primarily residing in the internal fissures of Apy2 as micron silver gold ore. The evolution characteristics of gold-bearing sulphide and the solubility curve of Au indicate that gold mainly derives from gold-bearing unsaturated fluid, rather than from early framboidal pyrite Py0 (0.52–1.06 ppm, average 0.79 ppm). Only a minor fraction of gold (originating from early sediments) in Py0 was liberated and integrated into subsequent pyrites. The content of antimony in Py0 significantly reduces after dissolution, suggesting that antimony predominantly originates from Py0 (206–440 ppm, average 335 ppm). Throughout the genesis of the Buzhu gold (antimony) deposit, the gold constituent underwent a sequence of pre-enrichment, re-enrichment, liberation, and precipitation. The processes of coupled dissolution-reprecipitation (CDR) and boiling fostered the reactivation and reenrichment of gold. The recrystallization into cubic pyrite led to the expulsion of gold from the pyrite lattice, and fluid oxidation resulting from multistage fluid boiling, coupled with a decrease in sulfur fugacity, led to gold precipitation.

Mapping hydrothermal alteration in regolith using white micas and chlorite as vectors towards gold mineralization

Mineral exploration through regolith-dominated terrains poses a significant challenge to cost-effective exploration techniques. Due to missing surface expression, undercover mineral exploration relies on understanding ore-forming processes and characterizing alteration regimes to decipher suitable vectors towards ore deposits. The work presented focuses on the Archean granite-greenstones of the Yilgarn Craton, east of the Meekatharra area in Western Australia, and characterizes the weathering profiles by understanding metal dispersion mechanisms and identifying mineralogical vectors towards gold mineralization within regolith. Mineral mapping of gold mineralization using TESCAN Integrated Mineral Analyzer and laser ablation ICP-MS shows gold is associated with multiple generations of pyrite, and base metal sulfides and sulfosalts hosted in felsic to intermediate volcanics and volcaniclastics. Intensive weathering generated a thick regolith profile dominated by a leached zone of kaolinitic and micaceous saprolite underlain by a supergene Au-Cu deposit blanket at the base. The supergene deposit is dominated by colloform and framboidal pyrite, with pure microcrystalline Au, chalcocite, bornite, malachite, and alunite. Hyperspectral analyses were used to trace the composition and abundance of chlorite and white mica variations in the host rock and the weathering profile. The mineral assemblage in the hydrothermal alteration halo proximal to and intersecting gold mineralization is dominated by Fe-rich chlorite and Na-rich white mica (paragonite). Fe-rich chlorite and paragonite are spatially tied to elevated Au concentration and trends to Fe-Mg-rich chlorite and K-rich white mica (muscovite) distal to the alteration. The variations in chlorite chemistry were detected mainly in bedrock and saprock. Conversely, the white mica chemistry variations were detected in bedrock and the regolith profile, in which white micas resist intensive weathering. The spectral signatures identified through short-wave and thermal infrared data are verified through X-ray diffraction, mineral chemistry, and bulk geochemical analyses. This distinctive spectral signature of white mica and chlorite is a cost-effective exploration method for regional mapping of mineral systems to identify hydrothermal alteration footprints in the regolith developed over felsic and intermediate rocks.

Impact of temperature on sphalerite chemistry: Simulation experiments of sphalerite in MVT Zn-Pb sulfide deposits in the Beiba Dome, northern Sichuan Basin, China

Many sedimentary metal deposits are rich in organic matter, and some organic deposits are metal deposits themselves. These metal deposits, represented by MVT Zn-Pb deposits, are widely distributed in the world, and their industrial types (Pb + Zn > 5 %) are also very rich, which has important theoretical and economic significance. However, there is no systematic research on the metallogenic mechanism of MVT Zn-Pb deposits in the participating organic matter. Therefore, geochemical simulation experiments and thermodynamic calculations have been used to examine the role of organic matter in reconstructing the ore-forming process of MVT Zn-Pb sulfide deposits. Additionally, the precipitation chemistry of sphalerite under the mineralization process mediated by organic matter is explained. Guided by the evidence on the homogenization temperature of fluid inclusion, the temperature range, from 60 °C to 200 °C, has drawn a close relationship between the temperature and the ore-forming mechanism. Notably, this reflects sphalerite’s chemical composition and morphology, even assisting in interpreting its chemical formation. The results showed that 80 °C is the beginning temperature for ZnS formation calculated from the inductively coupled plasma mass spectrometry analysis (ICP-MS) under the precipitation simulation experiments with the participation of organic matter. With the increased temperature, the rate of ZnS formation showed a trend of rapid growth in the early stage (80–140 °C) and relatively slow in the later stage (160–200 °C). This finding suggests a closer coupling relationship between metal mineralization and the maturation-genesis of organic matter, mainly the formation of the ancient oil reservoir. Following the continued increase in temperature, the morphology of ZnS was significantly different from that of the scanning electron microscope (SEM), with an apparent fine granular structure at high temperature. Likewise, more ZnO was also found through the X-ray photoelectron spectroscopy analysis (XPS), reducing the purity of the ZnS in sphalerite. Thus, this study helps reveal the metallogenic mechanism of MVT Zn-Pb sulfide deposits from the viewpoint of sphalerite chemistry under the participation of organic matter from temperature change. Furthermore, it provides a valuable theoretical and experimental basis for the later prospecting of the metal deposits.

Iron isotopic fractionation by magmatic-hydrothermal fluid-melt interaction in the Himalayan leucogranites

Iron (Fe) isotopes have been used to trace magmatic evolution (e.g., mineral fractional crystallization, partial melting degree, and fluid exsolution), but the behavior of Fe isotopes in magmatic-hydrothermal processes remains poorly constrained. Here we report Fe isotope compositions of the Kampa leucogranites from the Himalayan orogenic belt to understand the fluid-melt interaction and rare metal enrichment in highly-evolved granites. The Nb/Ta ratio divides samples into two groups: group 1 with high Nb/Ta ratios while group 2 with low Nb/Ta ratios. The δ56Fe of group 1 leucogranites range from 0.14‰ to 0.23‰, isotopically heavier than the average upper continental crust value, indicative of the residues from fractional crystallization. On the contrary, the group 2 leucogranites exhibit large range of δ56Fe from -0.21‰ to 0.21‰. The correlations between fluid-mobile/fluid-immobile element ratios, and iron isotope, and tetrad effect were attributed to the effect of magmatic-hydrothermal fluid-melt interaction during magma evolution. A fluid-melt interaction model with an initial δ56Fe of -0.4‰ for the fluid and a fractionation factor Δ56Fefluid-melt of -0.35‰ successfully explains the observed Fe isotope data. It is likely that these magmatic-hydrothermal fluids are derived from exsolution from the underlying magma reservoirs in the middle crust, which can modify the Fe isotope compositions and bring abundant rare metal elements into the leucogranites. Therefore, Fe isotope data can reveal the properties of magmatic-hydrothermal fluids and serve as an indicator for ore-forming processes.

Trace element and isotope composition of calcite, apatite, and zircon associated with magmatic sulfide globules

AbstractThe formation of volatile-rich phases in magmatic sulfide systems has been interpreted at least in six different ways. The most popular model attributes their origin to secondary processes, mostly due to the presence of serpentine, chlorite, phlogopite, amphibole, and calcite. While chlorite and serpentine are likely to form as alteration products, the other volatile-rich minerals have the potential to originate in a range of ways, including by primary magmatic processes. Based on mineralogical and petrological studies, it was recently suggested that volatile- and incompatible element-rich halos around sulfide globules may form due to the interaction between three immiscible liquids: silicate, carbonate, and sulfide. This hypothesis was confirmed by experimental data revealing the systematic envelopment of sulfide globules by carbonate melt, indicating their mutual affinity. In this study, we present data on isotopic signatures and trace element distributions of three minerals commonly found in spatial association with sulfides—calcite, apatite, and zircon—to address the question of the source and nature of volatiles and other incompatible elements involved in the formation of the halos. Here we compare our new hypothesis with all the previously proposed explanations to show if they can be consistent with obtained results. Our findings indicate that both mantle and crustal sources play a role in the formation of volatile- and incompatible element-rich halos, strongly correlating with sulfur isotope data previously reported for the sulfide globules in the same intrusions. This correlation confirms the shared origin of sulfides, carbonate and fluids during ore-forming processes, ruling out the secondary origin of volatile-rich phases. The isotope and trace element signatures support the newly proposed hypothesis that volatile- and incompatible element-rich halos could have been formed due to the interaction of immiscible sulfide, carbonate, and silicate melts. The volatile-rich carbonate melt could be sourced from the mantle or it could be added from the crust. Regardless of the origin, carbonate melt and sulfide liquid both immiscible with mafic magma tend to stick to each other resulting in the formation of volatile- and incompatible element-rich halos commonly documented in magmatic sulfide deposits.

Substantial in situ Ti isotope variations in rutile record source and fluid evolution of porphyry copper mineralization systems

Titanium (Ti) and its stable isotopes have been widely used as tracers for magmatic processes. However, our understanding of Ti isotope behavior in magmatic-hydrothermal systems remains limited. Hence, the in situ Ti isotope composition (δ49Ti) of magmatic titanite and hydrothermal rutile associated with magnetite and chalcopyrite mineralization was determined for the first time in four well-characterized porphyry copper deposits in southern Tibet. The rutile formed through the alteration of primary Ti-rich minerals during fluid-rock interaction in the early high-temperature magnetite and later moderate-temperature chalcopyrite stages of mineralization. Hydrothermal rutile, altered from magmatic titanite, exhibits δ49Ti values similar to those of residual magmatic titanite. This suggests that hydrothermal rutile inherited the Ti isotope composition of magmatic titanite. The average δ49Ti values of rutile are negatively correlated with whole-rock εNd(t) and zircon εHf(t) data, and positively correlated with whole-rock (87Sr/86Sr)i values, which suggests that the initial Ti isotope compositions of hydrothermal rutile in porphyry copper deposits primarily reflect their source. Rutile from the Qulong deposit sometimes exhibits fractionation of δ49Ti at levels exceeding 0.5‰, displaying a negative correlation with Zr and FeO, which may be attributed to the formation of magnetite and rutile at an early potassic alteration stage. Isotopically light Ti is preferentially incorporated into magnetite and rutile. Thus, the rutile associated with sulfide mineralization that formed from the remaining fluids during a later stage of phyllic alteration is enriched in heavy δ49Ti. These findings contribute to the understanding of how rutile fractionates Ti isotopes in hydrothermal systems related to porphyry copper deposits. In local contexts, the substantial crystallization of magnetite, along with the preferential incorporation of isotopically light Ti during the early stages, leads to a decrease in oxygen fugacity within the ore-bearing fluid. This, in turn, facilitates the formation of sulfides during later stages. The results of this study demonstrate the efficacy of in situ Ti isotope analysis as a powerful tool for tracking fluid and metal sources, and can be used to help interpret ore precipitation throughout different stages of magmatic-to-hydrothermal ore-forming processes.

Role of metasomatism in formation of the Yichun rare-metal deposit, China

What role post-magmatic processes have played in the development and mineralization of rare-metal peraluminous granites and how important that role can be, are questions that researchers have been wrestling with for decades. The Yichun Li-Ta-Nb deposit is an example that has been the subject of such a debate. The granitic appearance of rocks, the inert nature of tantalum, and the scarcity of mineralization in country rocks have been taken to suggest that fluid overprints were limited to within the granites and were unimportant in rock- and ore-forming processes. In this study, the identification of two types of abundant Li- and Cs-rich pseudomorphs in both the topaz-lepidolite granite and overlying pegmatite, the only two mineralized units at Yichun, suggests that extensive metasomatism was involved in rock and ore formation. The textural and chemical similarities of lepidolite in comparable mineral assemblages from a variety of occurrences, including lepidolite in pseudomorphs, veins, and miarolitic cavities from both the topaz-lepidolite granite and pegmatite, suggest that all lepidolite at Yichun is metasomatic and largely inherited its chemical signatures from a magmatic-hydrothermal transitional fluid, rich in Li, Cs, and Ta, derived possibly from the Li-muscovite granite that lies beneath the topaz-lepidolite granite. We propose that this transitional liquid, composition of which lies between a silicate melt and aqueous fluid, was not in equilibrium with the original igneous mineralogy, thus bringing about significant metasomatism along its infiltration and evolution upwards. Variations in the trace-element composition of lepidolite likely reflect the influence on mineral compositions by precursor minerals. The ambiguous boundary between the topaz-lepidolite and Li-muscovite granites, combined with the intensely metasomatized nature of the former, is most consistent with the topaz-lepidolite granite being the extensively altered upper portion of the Li-muscovite granite, which is itself somewhat metasomatized. Although magmatic fractionation played a key role in the initial concentration of Nb, Li, and Ta in both granite and pegmatite formation, the Li-Ta-Nb-Cs mineralization and its host rocks were largely formed through magmatic-hydrothermal rejuvenation and re-enrichment.

AGE AND GEODYNAMIC SETTINGS FOR FORMATION OF CARBONATITE COMPLEXES AND ASSOCIATED RARE METAL DEPOSITS OF THE SOUTH URAL

The Ilmen-Vishnevogorsky and Buldym carbonatite complexes occurring in the Southern Urals represent linear deformed carbonatite complexes. Their origin, as well as the age and geodynamic conditions remain the subject of debate. The isochron methods (Rb-Sr, Sm-Nd, TIMS) and local U-Pb-dating of zircons (SHRIMP II and LA-ICP MS) of these carbonatite complexes were employed to determine the age and duration of the stages of alkali-carbonatite magmatism and associated rare-metal ore formation. The Silurian-Devonian U-Pb zircon ages of the early phases of Ilmen-Vishnevogorsky miaskites and carbonatites were determined as 420.7±11 Ma (S2) and 417±2.8 Ma (D1), respectively. In the later phases of miaskites and carbonatites, early zircons are resorbed, they have broken isotope systems, and later zircon generations form a cluster of 386±7.6 Ma (D2). The Lower Permian U-Pb age of zircon 280±8 Ma (P1) was determined in the miaskite-pegmatite and late carbonatite. The isochron dating of late ore-bearing varieties of carbonatites provided the ages from the Lower Permian to the Early Triassic (P1–T1): 254±18 Ma, Sm-Nd and 247±4 Ma, Rb-Sr, IVC; 280±53 Ma, Sm-Nd, Buldym complex. Thus, the generation and intrusion of alkaline magmas in the Urals occurred ~420 Ma (S2–D1), synchronously with the formation of island-arc complexes. They are related to rifting on the emerging continental margins. The tectonic activity and formation of alkaline rocks and carbonatites proceeded in the Middle Devonian (~380 Ma, D2); it correlates with the accretion-collision stage of the Urals development. At the stage of "hard" collision (~280 Ma, P1), the Ilmen-Vishnevogorsky and Buldym complexes were plastically deformed, underwent melting and emplaced conformably with collision-slip tectonic structures. Recrystallization of rocks and minerals, plastic and brittle deformations, processes of pegmatitic, carbonatitic and rare-metal ore formation are associated with palingenic-metasomatic transformation of rifting alkaline complexes of Silurian-Devonian age at the collision and post-collision (~250 Ma, P3–T1) stages of the Urals emplacement.

Fluid inclusion and pyrite geochemistry of the Jiapigou gold deposit, North China Craton: Implication for origin of orogenic gold deposit?

The Jiapigou auriferous belt in Jilin province has been recognized as substantial gold-producers in the North China Craton (NCC). Gold mineralization in this district mainly occurs in mineralized quartz veins and alteration zones, characterized by silicification, pyritization, and argillitization. The quartz veins contain three generations of mineralized quartz, namely, qtz1 (probably oldest), qtz2 (slightly younger), and qtz3 (probably youngest) were identified using SEM-CL. The fluid inclusions in all three generations of quartz are broadly of three types, viz., bi-phase Ia (H2O-CO2), tri-phase Ib (H2O-CO2-NaCl ± CH4), and II (H2O-NaCl).The micro-thermometric analysis of the type Ia and Ib fluid inclusion in qtz1 & qtz2 have aqueous-carbonic composition and exhibit a similar salinity of 1.1 to 7.8 wt% NaCl equivalent, whereas the homogenization temperature (Th) range varies from ∼237 to ∼350 °C. These ore-related fluid inclusions are of low to moderate salinity, and show mixture of CO2 and CH4. On the other hand, Type II fluid inclusions are dominant in qtz3. They have salinity of 3.2 to 12.7 wt% NaCl equivalent and homogenization temperature varying between 180 °C and 210 °C. These data indicate that the ore-forming fluid evolved from a CO2–H2O–NaCl ± CH4 system during the mineralization period.The X-ray elemental maps of pyrite acquired using electron microprobe analysis (EPMA) show irregular zones of Co and Ni indicating two generations of pyrite. The early-stage pyrite which occupies core portion shows high Co and Ni, whereas the later-stage pyrite occupying rim has lower Co and Ni. The laser ablation-inductively coupled plasma-mass spectrometer (LA-ICPMS) analyses of pyrite has indicated that pyrite-1 is rich in Au + Ag + Se + Cu + Co + Ni, whereas, pyrite-2 has lower concentration of these elements. A positive correlation between Fe and other chalcophile elements reported here (i.e. Au + Ag + Se + Cu + Co + Ni), might be due to fluid-rock interaction resulting into a saturated fluid that subsequently precipitated along the microfractures within earlier-formed pyrite and quartz. The in-situ δ34S values in pyrite from Jiapigou deposits overlap in the range of +4.5 to +9.6 ‰, which is consistent with the ore-forming fluids of the crustal origin input during fluid-rock interaction. The systematic pyrite compositional observations and fluid inclusions study documented here to provide new insight into the process of ore formation for the Au enrichment in the Jiapigou deposit.

Basin inversion, drifting and hyper-extension in the Central Atlantic Ocean and Maghrebian Tethys as fluid driving mechanisms for the superimposed base metal mineralization events in the Jbel Bou Dahar carbonate-hosted Pb-Zn-Ba deposits (High Atlas, Morocco)

The Early Jurassic Jbel Bou Dahar reef-bearing carbonate platform in the eastern High Atlas, Morocco, contains numerous carbonate-hosted and fault-controlled Pb-Zn-Ba deposits, prospects, and showings. Combined field, petrographic and geochemical data, including REE + Y analyses, stable (C-O-S), radiogenic (SrPb) and noble gas (He, Ne, Ar) isotopic compositions, record two temporally distinct metallogenic events. Both events are related to post-rifting and inversion of the inherited Mesozoic Atlas paleorift. The ore deposits are thought to have been formed during large-scale episodes of transtensional basin evolution, which developed as a far-field effect of the drifting of the Central Atlantic Ocean and Maghrebian Tethys while the basin continued to invert in response to the ongoing convergence between the African and the Eurasian plates. The paragenetically earliest Zn-rich ore was generated in a transtensional setting coincident with the hyper-extension of Maghrebian Tethys and the initial stages of drifting and formation of the Central Atlantic passive margin. Crustal extension and high heat fluxes provided by the hot upwelling mantle would have triggered buoyancy-driven convection of deeply sourced fluids and maturation of organic matter dispersed within the host carbonates to produce hydrocarbons within an extensional basin setting. These relatively reduced Zn-rich ore-forming fluids acquired metals mainly through protracted interaction with the country rocks including the underlying Triassic siliciclastic series and the granitic basement rocks. Regional ENE- to E-W-trending major faults would have acted as conduits for upward fluid flow and traps for Zn-Pb-Ba mineralization. Conversely, the late Pb-rich mineralization stage II is broadly constrained to have occurred between ca. 70 Ma and 10 Ma and is thought to be related to the transition from orogenic shortening (late Eocene) to post-orogenic crustal extension and exhumation (early Miocene) following Alpine orogenic collapse. Mixing of down-ward percolating meteoric fluids and ascending rock-buffered hydrothermal brines that had previously equilibrated with the radiogenic basement rocks along with thermochemical sulfate reduction of transported sulfate and/or thermal cracking, would have been the main ore-forming processes that controlled ore deposition. Given these fluid characteristics and geodynamic settings, the Jbel Bou Dahar ore field is classified as a hybrid carbonate-hosted Pb-Zn-Ba district formed by the juxtaposition of two deposit types rather than one progressively evolving mineralizing system. From an exploration standpoint, new delineation here of the two metallogenic events provides valuable exploration tools for further targeting PbZn and barite resources in the Atlas system of North Africa and other areas where similar orogenic belts experienced post-rift subsidence and basin inversion.